Projects & Technical
Send Morse on your VHF Rig
by Peter Parker VK3YE - first appeared in Amateur Radio, June 2000
Everyone knows that the
best way to practice Morse is to use it on the air. But how do you send Morse if you don’t have a multimode HF or VHF
transceiver? Well, you could hold a
microphone up to a code practice oscillator, and hold the PTT down while
pressing the key, but it’s very clumsy, and the transmitted tone is likely to
be harsh. Clearly something better is
needed.
Enter
the MorseBox! It lets you send quality
Morse from a normal two metre or 70 centimetre FM transceiver. Just plug it in to the rig’s microphone
socket and you’re on the air. Using
just one transistor and a handful of other parts, the MorseBox can instantly be
switched between Morse and speech - a handy feature for those running Morse
practice sessions with readbacks.
MorseBox also includes semi-break-in to automatically switch between
receive and transmit when the key is pressed and a sidetone to allow monitoring
of keying.
Circuit Description
MorseBox
consists of two stages. These are a
tone oscillator and a transceiver control/timing circuit (Figure One).
The
tone oscillator provides a pure tone to modulate the transmitter. It is turned on and off by keying the
emitter circuit. A twin-T circuit was
used because it is reliable and reproducible.
The output is fed to the transceiver microphone connection via a
variable resistor which is adjusted to provide a sufficient audio level for the
rig used.
The
control and timing circuit tells the transmitter when to transmit and when to
receive. Pressing the key activates
the relay and causes the rig to switch to transmit. So that the transmitter does not drop out between individual dits
and dahs, a large capacitor has been wired across the relay to provide a delay
of several seconds. This means that the
carrier is on constantly and makes for more comfortable reception. If the key hasn’t been pressed for several
seconds, the relay drops out and the transceiver switches to receive.
The
delay time depends on the value of the capacitor and the resistance of the
relay coil. Slower senders will prefer
a longer delay and faster operators will want a shorter delay. The prototype had a delay of approximately
three seconds, which should suffice for Novice speeds. Substituting a smaller value capacitor or
lower resistance relay will lessen the delay (and vice-versa). Experimentation may be required to produce
an appropriate delay from the components at hand.
The
front-panel switch allows Voice or Morse to be selected. When it is switched to voice, the MorseBox
is disabled, and the hand microphone is connected straight to the transceiver. When Morse is selected the audio from the
microphone is cut off and power is applied to the audio oscillator and relay
circuits.
A
useful feature is the sidetone or keying monitor. This allows you to monitor your own sending without needing a
separate receiver. Though an IC audio
amplifier and speaker could be used, this would increase the project’s
complexity. Instead it was decided to
use a small piezo buzzer wired between the supply rail and key as sidetone. The 10k series resistor reduces the buzzer’s
volume and extends battery life.
Construction and testing
House
the project in a metal case. This is
important to prevent the transmitted signal feeding back into the audio
oscillator and distorting the tone. The
case pictured is a 100x77x130mm box by K&W.
All
components except for the capacitor across the key socket, switch, sockets and
battery holder are mounted on a piece of unclad perforated circuit board. The parts were a tight squeeze on the
60x80mm board used in the prototype.
75x76mm boards are commercially available and would have allowed a
better layout. Component leads are
passed through the board and are soldered underneath. Vero-type stripboard could be used, but allows a less flexible
layout than the blank matrix board recommended. Because builders will use different component and circuit board
types, no component layout diagram is provided. Instead treat this project as an opportunity to build something
straight off a circuit diagram - an important amateur skill as discussed in
April’s Novice Notes.
Before
commencing construction, work out where components will be placed on the
circuit board. This is particularly
important for the larger parts, such as the relay, electrolytic capacitors and
piezo buzzer. Allow space for a hole near
each corner of the board to accommodate each spacer. Don’t forget to connect the case to the supply negative
line. This can be done in several
places - most conveniently through the key socket.
Use
a 6.5mm mono headphone socket for the key.
The only other socket on the rear panel is the connection to the
transceiver. This connection should
have sufficient pins to suit your transceiver’s microphone connections. An eight pin microphone socket and
detachable patch lead was used in the prototype, but if you’re really hard-up,
this can be replaced with a flying lead with plug to suit the transceiver.
Drill
two holes in the front panel. One is
for the Morse/Voice switch and the other for the microphone connection. No socket was used for the microphone
connection as the ex-commercial microphone used was cheap enough to be
sacrificed full-time for this project.
However one should be used if you wish to use your transceiver’s normal
microphone on the MorseBox. Don’t
forget the ferrite bead on the connection to the hand microphone - this reduces
the risk of transmitted RF getting in to the audio.
Install
the wiring around the Speech/Morse switch, relay contacts, the microphone and
the socket that carries the connections to the transceiver. Take care as this wiring quite complicated
and it’s easy to make a mistake. Trust
me, you’ll almost certainly get it wrong first time! Use the testing process described later to check for such faults
before the MorseBox is wired to the transceiver.
The
power supply used is up to the builder.
Batteries were used in the prototype to make the unit fully portable and
eliminate the need for an external supply.
A bank of six ’AA’ was used in the prototype. This is a good compromise between battery life, size and
cost. The small nine volt batteries
could be used, but have limited capacity and may give rise to oscillator chirp
near the end of their lifespan.
Schematic Diagram of MorseBox
Connecting MorseBox to your transceiver
To
connect the MorseBox to your transceiver you will need to make a cable. Because the required connections vary
between transceivers, it is not possible to provide the details here. Instead you will need to study the
microphone connections as given in your transceiver’s user manual or schematic
diagram. Identify the microphone’s
basic connections (ground, microphone audio and push-to-talk) and note any other
leads that may be present.
Transceiver
microphones are full of traps for the unwary.
Some PTT buttons do more than just key the transceiver - check for
multi-section switches that have other functions such as disconnecting the
microphone element when receiving. Also
many microphones have up-down buttons and other functions that require extra
wires. These wires should be provided
for in the cable between the transceiver and the MorseBox if these extra
features are to be available when the MorseBox is connected.
Be
prepared for the possibility that both sides of your rig’s PTT connection will
be floating above earth. This will
affect the way the Morse box is wired to the transceiver. More specifically, the NO terminal of the
relay and the earth side of the microphone’s PTT (as connected through the
front panel socket) should be disconnected from earth and wired directly to the
(formerly earthed) side of the PTT.
Connecting
the MorseBox to the transceiver is the hardest part of the project for the
newcomer. This is especially for transceivers
with complicated microphone connections.
Seek assistance from a more experienced amateur if in doubt - in extreme
cases a wrong connection may damage the transceiver and void the warranty.
Testing and Adjustment
Test
the audio oscillator by applying power, switching to Morse and pressing the
key. Set the 10k trimmer potentiometer
to about half-position. Connect a pair
of high impedance headphones, a crystal earphone or an audio signal tracer to
the lead carrying audio to the transceiver socket on the back panel. If all is well you’ll hear an audio tone
while the key is down.
If
nothing is heard, a wiring fault is likely.
Firstly check that the top end of the 3.9k resistor is +9 volts relative
to earth. If not, look for wiring errors
near the Speech/Morse switch. Wrong
connections in this area could also mean the oscillator is working but audio is
not getting to the transceiver socket.
Other reasons for failure include the transistor and the diodes being
wrongly connected.
If
you’ve wired in the buzzer, a sound from this should also be heard when the key
is pressed. If no sound, check the
buzzer’s polarity.
Pay
attention to the relay’s action.
Observe it pull in as soon as the key is pressed. The relay should remain in for about three
seconds after the key is released. In
Morse mode the relay controls the transceiver’s PTT. The three-second delay should be long enough to keep the
transmitter keyed down between Morse letters.
Connecting an audible continuity indicator (such as that found in many
multimeters) across the earth and PTT terminals on the rear panel socket should
result in a continuous tone while the key is being pressed, only dropping out
when three seconds have elapsed after the key was last touched.
Using MorseBox
The
Morse Box can be left in the transceiver’s microphone lead at all times. The ‘speech’ setting allows normal voice
operation, while the Morse setting allows Morse to be sent when the key is
pressed.
The
MorseBox has been set up for semi-break in operation with automatic switching
from transmit to receive. Operators can
switch to voice at any time with the Morse/Voice switch. This is useful if doing readbacks after text
has been sent.
Most
people would be satisfied with using the Morse Box on a two metre repeater or
simplex frequency. However, crossband
operation (possibly using ten metres, six metres or 70 centimetres) can assist
communication. This is because
receiving stations could ask for repeats or request faster sending while the
other station is sending. The effect
would be akin to the full-break-in enjoyed by proficient HF CW operators.
Parts List
(DSE catalogue numbers given for
convenience)
BC548
NPN transistor 1 Z1308
1N4148
diodes 2 Z3120
3.9k
1/4 watt resistor 2 R1088
5.6k
1/4 watt resistor 2 R1092
10k
1/4 watt resistor 1 R1098
270k
1/4 watt resistor 1 R1134
10k
trimpot 1 R1941
1nF
disc ceramic capacitor 2 R2307
10nF
greencap capacitor 5 R2055
100nF
greencap capacitor 1 R2100
220uF
electrolytic capacitor 1 R4380
6800uF
electrolytic capacitor 1 R4470 (see text)
6-9v,
220 ohm SPDT relay 1 P8008 suggested (see text)
Small
piezo buzzer 1 L7020
DPDT
toggle switch 1 P7656
Hand
microphone with PTT 1 (see text)
8-pin
microphone socket 1 P1826
8-pin
microphone plug 1 P1836
Mic
plug to suit transceiver 1 -
6.5mm
mono socket 1 P1261
6xAA
battery holder 1 S6116
Snap
to suit battery holder 1 S6100
100x77x130mm
metal case 1 H2804
75x76mm
blank matrix board 1 H5310
10mm
insulated spacers 4 H1861
Ferrite
bead 1 R5425
Conclusion
A
device to allow the transmission of Morse on VHF/UHF FM-only transceivers has
been described. It is simple to build and
provides an easy way for amateurs to practice Morse on the air. It would also be an ideal club or group
project for those wishing to increase their Morse skills together.
back to Gateway Projects
& Technical Page
This page was produced by Peter Parker VK3YE parkerp@alphalink.com.au. Material may be copied for personal or non-profit use only.